1. Field of the Invention
This invention relates to improvements in resistance wire strain gage type transducers. More particularly, the invention is directed to an improvement in transducers of the resistance wire type in which the resistance element (which will normally be referred to as a "wire" or "filament" herein, since this is the usually preferred form of resistance element used) itself serves not only as a sensor to measure the strain, but is also a structural member or element so that the resistance wires are subjected to and support the entire load in the transducers.
Strain gages of the resistance wire or filament type have been used for some time in combination with various mechanical devices, such as, links, tubes, rings, etc., for measuring variable quantities, such as, strain, pressure, torque, acceleration, etc. The typical resistance wire strain gage type transducer contains a resistance wire or filament connected in a conventional four-arm bridge circuit, which, ordinarily, is balanced. The force to be measured is usually coupled mechanically to a link or ring or other device on which gages are bonded or attached by other means. The force exerted on the link introduces strains in the wire through the coupling means and causes the dimensions of the wire to be altered which results in a change in the resistance of the wire proportional to the load in the link. This, in turn, causes unbalancing of the bridge in an amount at least roughly proportional to the force applied to the link.
Except for certain units designed to measure very small loads (e.g., of the order of grams or ounces) most of the strain gages of the resistance wire or filament type which are now in use are not mechanically coupled directly to the force to be measured, and do not measure directly the total force. Instead, it is customary to mount a resistance wire type of strain gage on the surface of the test specimen or structural member in which it is desired to measure the strain imposed on that test specimen or structural member.
At the present time, two different methods are used principally for mounting the resistance wire or resistance element in most gages of this type. The first method is commonly referred to as the "bonded method." In accordance with this method, the resistance wire is bonded by means of cement, glue, or similar adherent to the surface of a sheet of an insulating material, usually paper, although plastic materials or plastic coated metal may be used. The ends of the wire are adapted to be connected, usually through lead wires, to form one arm of a conventional four-arm bridge circuit. The insulating material or paper is, in turn, adapted to be bonded by an adherent or welding to a test specimen in which strain is to be measured. Strain applied to the specimen is transmitted through the bonded paper or other supporting sheet material to the resistance wire, and the resulting change in the dimensions and resistance of the wire is indicated by the relative degree of unbalancing of the bridge circuit. Suitable calibration or tables provide means for measuring the amount of strain, force, pressure, or the variable which the device is designed to measure.
The second method for mounting a resistance wire for use in such gages is the so-called "unbonded method." In the unbonded method, the strain wire, that is, the resistance wire, is stretched around insulating supports, mounted on the test specimen and which are adapted to be made movable with respect to each other in response to a strain introduced into the test specimen to which the gage is attached.
In both of these methods, since the resistance wire does not support the total force applied to the test specimen, it is necessary to carefully position the resistance wire element on the test specimen in order to be sure that it is so positioned that it accurately reflects the total strain or force imposed on the test specimen. In many instances, it is necessary to use a plurality of resitance wire elements positioned at several points on the test specimen to be sure that the maximum force to which that test specimen is subjected is measured.
It is, also, well known that the power sensitivity of a Wheatstone bridge and most other circuits employed in the measurement of resistances is directly proportional to the magnitude of the current which can be carried by the resistance circuit being measured and the voltage which can be impressed on the circuit. In view of this, it is apparent that the sensitivity of measurement of strain by means of a resistance wire type of strain gage is directly proportional to the current which can be passed through the strain sensitive element of the gage and the voltage. These, in turn, are proportional to a combination of the size or diameter of wire of filament used in the strain sensitive element and the length of wire. It is also proportional to the permissible operating temperature and the effective heat dissipating capacity of the gage element as a whole when attached in readiness for use to the member or test specimen to be stressed.
In most of the strain sensitive elements employed in the strain gages used in both the bonded and unbonded methods referred to above, the resistance wire or filament is of extremely small diameter and, thus, the current carrying capacity of such strain sensitive element has been so limited as frequently to require the employment of some method of amplification in conjunction with the Wheatstone bridge circuit in order to attain the requisite high sensitivity of the strain measurement and power to drive a meter. It has also resulted in the strain sensitive element being rather fragile or delicate and thus readily damaged or destroyed in use or handling. In addition, the direct force bearing capability is minute.
It is, accordingly, an object of this invention to provide an improved strain gage of the resistance wire type which is of particularly rugged and sturdy construction and which, if desired, can be subjected to the entire force to be measured, and function both as the load bearing element and sensor.
It is a further object of this invention to provide an improved transducer of the resistance wire type having increased current carrying capacity and power sensitivity over those heretofore employed. Some models can be used to operate standard meters, relays and other devices with no amplification.
A further object is to provide a strain gage having good heat dissipating qualities.
A further object is to provide an improved transducer of sturdy and simple construction.
Other and further objects of the invention will be apparent as the present description progresses.
In brief, the strain gage of the present invention comprises two spaced apart load carrying or attaching devices which are connected by a plurality of parallel resistance wires. While the resistance wires connecting the two load carrying or load attaching elements are mechanically parallel and closely grouped or packed, they preferably are in series electrically so as to form a continuous length of resistance wire passing back and forth between the two load attaching or carrying elements.
The details of the present invention will be apparent to those skilled in the art from a consideration of the accompanying drawings which illustrate preferred embodiments of the invention, and the description thereof which follows.